The present invention relates to security systems, for example, for tracking patients in a hospital or medical care facility, and in particular to a security system providing improved range discrimination when wirelessly interrogating patient-held transponders.
Security systems may be used m institutions such as hospital for monitoring patients, for example in pediatrics to deter infant abductions, and in nursing homes or neurological centers to reduce the likelihood of an Alzheimer's or head trauma patient wandering out of the facility.
Wireless security systems equip each patient with a small, wearable transponder (transmitter and receiver). The transponder monitors a selected frequency to detect an activation signal and is activated when an activation signal on the selected frequency is detected at a predetermined threshold amplitude. The activation signal originates from a control unit with an antenna typically located near a door or other exit. In this way, as the patient approaches the door exit, the amplitude of the received activation signal rises until the transponder is activated.
Upon activation, the transponder transmits an identification signal to a receiver connected to the control unit and a control unit decodes the identification signal to lock or unlock the door.
One disadvantage to such a wireless system is that it can be hard to localize the wireless identification signal to a particular door. One reason for this difficulty is that the receiving antenna on the patient transponder may have a variety of different orientations which significantly change its sensitivity as it varies from the orientation of the activation signal. As a result of this variation in practical sensitivity, the activation signal needs to be set to a power level that ensures activation of the patient transponder even with the worst-case transponder antenna orientation. As a result, the activation signal can activate some transponders at a substantial distance from the door including, for example, in rooms or corridors so far removed from the door so as not to warrant activation of the control unit. The difficulty of precise localization of a region of activation can limit the use of such security systems in many important applications where such false triggering cannot be tolerated.